Communication systems using multiple antennas at both the transmitter and the receiver have been developed. Systems that utilize multiple transmit and receive antennas may be referred to as multiple-input multiple-output (MIMO) systems. The multi-antenna configurations may be utilized to mitigate the negative effects of multipath and signal interference on signal reception. With the introduction of downlink MIMO, a wireless transmit/receive unit (WTRU) may receive multiple data streams simultaneously on the same frequency.
In high speed downlink packet access (HSDPA), downlink transmissions are scheduled by the Node B in a 2 ms transmission time interval (TTI) basis. In many cases there is not enough data for a single user to fully fill a 2 ms TTI. Internet traffic studies have shown that quite a large number of packets are in the order of 2 or 4 kbits, topped with the downlink traffic for cases like signaling radio bearer (SRB), voice over IP (VoIP) or transmission control protocol/Internet protocol (TCP/IP) acknowledgements for uplink traffic.